Write examples with larger arrays

[riking]

All of the array sizes in the documentation examples seem to be fairly small: the element type is always bytes, and the size is never more than about 5. I think that an example of hash function round constants would be more compelling, such as the data found here:

https://github.com/jedisct1/rust-hmac-sha256/blob/e787a2e40ce00c7e27b336207cdec730226ff6f1/src/lib.rs#L127-L138

Here, the program needs 64 u32s and loads (up to) 16 at a time.

Motivated via rust-lang/rust#109000.

[Cryptjar]

Certainly, a more elaborate example would be a welcome addition. Tho, an entire HMAC implementation seems a bit excessive.
If you have a good idea for an example with a more 'practical' array demonstration that isn't worth an entire crate on its own, I would gladly accept your PR.

[n-eq]

Hello. for what it's worth, here is a "real-world" example of using the library.
progmem is used to store a 504 bytes bitmap that is drawn to an 84x48 LCD screen.